In order to power various electrical components, internal combustion engine driven vehicles such as motorcycles and buggies are equipped with a power supply unit comprising an AC generator that is rotatably driven by the internal combustion engine and a battery that is charged by the output of the generator, the output of the power supply unit being used to supply electrical current to loads such as headlamps. In order to keep the battery in a suitably charged state, vehicles equipped with a power supply unit of this sort are equipped with a power supply unit control device comprising a battery charging control unit for controlling battery charging so as to keep battery terminal voltage at a set value while the internal combustion engine is running and a conducting control unit for controlling load conducting.
A magnetic AC generator provided with a magnetic rotor in which a magnetic field is formed by a permanent magnet and a stator having a magneto coil is usually used as the AC generator installed in internal combustion engine driven vehicles such as motorcycles and buggies. The magnetic AC generator has droop characteristics such that output voltage decreases when load current increases. Thus, in a vehicle in which a headlamp is illuminated by the output of a power supply unit provided with a magnetic AC generator driven by an internal combustion engine and a battery charged by the output of the unit, if a high rated consumed current is supplied to the headlamp when the engine is operating at low speed or idling, the output voltage of the magnetic AC generator is greatly decreased, becoming less than the battery voltage. In this state, a charging current cannot be supplied from the generator to the battery; thus, a high level of current is sent from the battery to the headlamp without the battery being charged, drastically draining the battery. If such a situation occurs frequently, there is a risk of the battery becoming over-discharged.
In the power supply unit control device for a internal combustion engine driven vehicle disclosed in Patent Document 1, a lamp driving switch is provided between the battery and the headlamp, and on/off control of the lamp driving switch is performed, thereby performing pulse width modulation (PWM) control of the lamp-driving current. In such a configuration, setting the PWM control duty ratio to a low value so as to limit the current flowing from the battery to the headlamp when the engine is running at low speed or idling, during which time the battery is not charged, limits the discharge current, suppressing battery drainage.